A. C. Gossard

University of California, Santa Barbara, Santa Barbara, California, United States

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Publications (912)2351.83 Total impact

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    ABSTRACT: Collective vibrations of proteins, rotations of small molecules, excitations of high-temperature superconductors, and electronic transitions in semiconductor nanostructures occur with characteristic frequencies between 1 and 10 THz. Applications to medicine, communications, security and other fields are emerging. However, mapping the coldest parts of the universe has been the largest driver for developing THz detectors. The result is a family of exquisitely-sensitive detectors requiring sub-4K temperatures. For earthbound THz science and technology, sensitivity remains important but many applications require high speed and operating temperatures. Room-temperature Schottky diodes enable some of these applications. Here we demonstrate a new type of detector in which THz radiation excites a collective oscillation of ~25,000 electrons between two gates in a microscopic four terminal transistor. The energy dissipates into other modes of the electron gas, warming it and changing the source-drain resistance. The detector shows amplifier-limited rise times near 1 ns and has detected THz laser radiation at temperatures up to 120K. The frequency of the collective oscillation tunes with small gate voltages. The first-generation tunable antenna-coupled intersubband Terahertz (TACIT) detectors tune between 1.5 and 2 THz with voltages <2V.
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    ABSTRACT: Advances in thin film growth technology have enabled the selective engineering of material properties to improve the thermoelectric figure of merit and thus the efficiency of energy conversion devices. Precise characterization at the operational temperature of novel thermoelectric materials is crucial to evaluate their performance and optimize their behavior. However, measurements on thin film devices are subject to complications from the growth substrate, non-ideal contacts, and other thermal and electrical parasitic effects. In this manuscript, we determine the cross-plane thermoelectric material properties in a single measurement of a 25 μm InGaAs thin film with embedded ErAs (0.2%) nanoparticles using the bipolar transient Harman method in conjunction with thermoreflectance thermal imaging at temperatures up to 550 K. This approach eliminates discrepancies and potential device degradation from the multiple measurements necessary to obtain individual material parameters. In addition, we present a strategy for optimizing device geometry to mitigate the effect of both electrical and thermal parasitics during the measurement. Finite element method simulations are utilized to analyze non-uniform current and temperature distributions over the device area as well as the three dimensional current path for accurate extraction of material properties from the thermal images. Results are compared with independent in-plane and 3ω measurements of thermoelectric material properties for the same material composition and are found to match reasonably well; the obtained figure of merit matches within 15% at room and elevated temperatures.
    Journal of Applied Physics 07/2014; 116(3):034501-034501-9. · 2.19 Impact Factor
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    ABSTRACT: Indirect excitons in coupled quantum wells are long-living quasi-particles, explored in the studies of collective quantum states. We demonstrate, that despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons, but also one of free electrons by two orders of magnitude.
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    ABSTRACT: We present new high-resolution measurements of transient time-domain photoconductivity in ErAs:InGaAs superlattice nanocomposites intended for THz photoconductive switches and photomixers using a pure optical pump-probe method. We developed a model, using separate photocarrier trapping, recombination, and thermal reactivation processes, which very accurately fits the measurements. The measured material structures all exhibit a slow secondary decay process, which is attributed to thermal reactivation of the trapped carriers, either into the conduction band, or into high-energy defect states. We examined the influence of superlattice structure, dopants, DC bias, and temperature. Analysis shows that all of the THz energy produced by the photocarrier trapping and decay processes are at frequencies less than 1 THz, while the reactivation process only serves to create a large portion of the bias power dissipated. Energy higher than 1 THz must be created by a fast generation process or band-filling saturation. This allows pulsed THz generation even from a long-lifetime material. Pure optical pump-probe measurements are necessary to expose slow material processes, and eliminate the influence of electrical terminals and THz antennas. These measurements and modeling of THz photoconductive devices are necessary in order to optimize the output spectrum and power.
    Journal of Applied Physics 07/2014; 116(1):013703-013703-8. · 2.19 Impact Factor
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    ABSTRACT: We report on self-assembled ErSb nanowires in a GaSb matrix that show a strong polarization-sensitive THz response. The nanowires behave like a polarizer. Their orientation and shape can be engineered by the growth conditions.
    CLEO: Science and Innovations; 06/2014
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    ABSTRACT: Transport, relaxation, and correlation effects are observed for indirect excitons in high magnetic fields.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: We experimentally demonstrate an order of magnitude higher radiated power from a 1550 nm photomixer with plasmonic contact electrodes in comparison with an analogous photomixer without plasmonic contact electrodes in the 0.25-2.5 THz frequency range.
    CLEO: Science and Innovations; 06/2014
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    ABSTRACT: We report the observation of spin currents and spin polarization textures in opti- cally generated indirect excitons. The textures are observed in linear and circular polarizations and are controlled by magnetic fields.
    CLEO: QELS_Fundamental Science; 06/2014
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    ABSTRACT: While the growth of III-As and III-P semiconductors is well-established, and their transport properties well-understood, the performance of high-frequency and VLSI electron devices can still be substantially improved. Here we review design principles, experimental efforts, and intermediate results, in the development of nm and THz electron devices, including nm InAs/InGaAs planar MOSFETs and finFETs for VLSI, InGaAs/InP DHBTs for 0.1-1 THz wireless communications and imaging, and ~5nm InAs/InGaAs Schottky diodes for mid-IR mixing.
    2014 72nd Annual Device Research Conference (DRC); 06/2014
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    ABSTRACT: We present Silver-epoxy filters combining excellent microwave attenuation with efficient wire thermalization, suitable for low temperature quantum transport experiments. Upon minimizing parasitic capacitances, the attenuation reaches >100 dB above ~150 MHz and - when capacitors are added - already above ~30 MHz. We measure the device electron temperature with a GaAs quantum dot and demonstrate excellent filter performance. Upon improving the sample holder and adding a second filtering stage, we obtain electron temperatures as low as 7.5 +/- 0.2 mK in metallic Coulomb blockade thermometers.
    Applied Physics Letters 03/2014; 104(21). · 3.52 Impact Factor
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    ABSTRACT: Optical control of exciton fluxes is realized for indirect excitons in a crossed-ramp excitonic device. The device demonstrates experimental proof of principle for all-optical excitonic transistors with a high ratio between the excitonic signal at the optical drain and the excitonic signal due to the optical gate. The device also demonstrates experimental proof of principle for all-optical excitonic routers.
    Applied Physics Letters 03/2014; 104(9):091101. · 3.52 Impact Factor
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    ABSTRACT: Multielectron spin qubits are demonstrated, and performance examined by comparing coherent exchange oscillations in coupled single-electron and multielectron quantum dots, measured in the same device. Fast (>1 GHz) exchange oscillations with a quality factor Q∼15 are found for the multielectron case, compared to Q∼2 for the single-electron case, the latter consistent with experiments in the literature. A model of dephasing that includes voltage and hyperfine noise is developed that is in good agreement with both single- and multielectron data, though in both cases additional exchange-independent dephasing is needed to obtain quantitative agreement across a broad parameter range.
    Physical Review Letters 01/2014; 112(2):026801. · 7.73 Impact Factor
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    ABSTRACT: We present measurements of the electron temperature using gate defined quantum dots formed in a GaAs 2D electron gas in both direct transport and charge sensing mode. Decent agreement with the refrigerator temperature was observed over a broad range of temperatures down to 10 mK. Upon cooling nuclear demagnetization stages integrated into the sample wires below 1 mK, the device electron temperature saturates, remaining close to 10 mK. The extreme sensitivity of the thermometer to its environment as well as electronic noise complicates temperature measurements but could potentially provide further insight into the device characteristics. We discuss thermal coupling mechanisms, address possible reasons for the temperature saturation and delineate the prospects of further reducing the device electron temperature.
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    ABSTRACT: We report on Terahertz (THz) detectors based on III-V high-electron-mobility field-effect transistors (FET). The detection results from a rectification process that is still highly efficient far above frequencies where the transistor provides gain. Several detector layouts have been optimized for specific applications at room temperature: we show a broadband detector layout, where the rectifying FET is coupled to a broadband logarithmic-periodic antenna. Another layout is optimized for mixing of two orthogonal THz beams at 370 GHz or, alternatively, 570 GHz. A third version uses a large array of FETs with very low access resistance allowing for detection of very short high-power THz pulses. We reached a time resolution of 20 ps.
    Proc SPIE 10/2013;
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    ABSTRACT: Quantum-dot spin qubits characteristically use oscillating magnetic or electric fields, or quasi-static Zeeman field gradients, to realize full qubit control. For the case of three confined electrons, exchange interaction between two pairs allows qubit rotation around two axes, hence full control, using only electrostatic gates. Here, we report initialization, full control, and single-shot readout of a three-electron exchange-driven spin qubit. Control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in less than 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements and non-orthogonal control axes.
    Nature Nanotechnology 09/2013; · 31.17 Impact Factor
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    ABSTRACT: We present both chip-scale and free space coherent arrays of continuous-wave THz photomixers. By altering the relative phases of the exciting laser signals, the relative THz phase between the array elements can be tuned, allowing for beam steering. The constructive interference of the emission of N elements leads to an increase of the focal intensity by a factor of NxN while reducing the beam width by ~1/N, below the diffraction limit of a single source. Such array architectures strongly improve the THz power distribution for stand-off spectroscopy and imaging systems while providing a huge bandwidth at the same time. We demonstrate this by beam profiles generated by a free space 2x2 and a 4x1 array for a transmission distance of 4.2 meters. Spectra between 70 GHz and 1.1 THz have been recorded with these arrays.
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    ABSTRACT: We demonstrate two-axis control of an exchange-only spin qubit in a GaAs triple quantum dot using a resonant microwave excitation. The qubit is operated in a regime where two separate exchange interactions are active simultaneously, suppressing leakage out of the qubit subspace and providing some immunity to charge noise. Spectroscopic probes of the qubit reveal that the resonance frequency can be adjusted between 100 MHz and 1.5 GHz with a voltage applied to the middle quantum dot. We find a coherence time T2˜20 μs for a 64 pulse Carr-Purcell-Meiboom-Gill dynamical decoupling sequence. Finally, analysis of the coherence time for multiple sequences reveals a power spectrum S(φ)˜&-0.9circ;, which suggests that the fluctuating Overhauser fields are not the dominant source of dephasing in this system.
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    ABSTRACT: Incorporation of erbium during MBE growth of GaSb leads to various self-assembled, semi-metallic ErSb nanostructures. At the lowest concentration, spheres of diameter 4-5 nm are observed. By contrast, at 7-10% Er, ˜5 nm diameter nanowires self-align along the <001> growth direction, and at 15-20%, the nanowires align in the growth plane along the <110> direction. Light polarized along the wires is strongly attenuated over a broad range from THz to near-IR. By contrast, light polarized perpendicular to the wires experiences minimal attenuation apart from a very strong surface plasma resonance at 0.46 eV. Surprisingly, the resonant frequency of the nanospheres is slightly higher than that of the wires, despite the smaller depolarization factor. Motivated by this observation and estimates of the confinement energy, we construct an effective medium theory for the nanostructures which includes a single characteristic intersubband transition. This model provides an excellent description of the IR reflectance and transmittance over the whole range of Er concentration, in contrast to a model which excludes the effect of quantum confinement.
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    ABSTRACT: We study finite-time Landau-Zener transitions at a singlet-triplet level crossing in a GaAs double quantum dot, both experimentally and theoretically. Sweeps across the anticrossing in the high driving speed limit result in oscillations with a small visibility. Here we demonstrate how to increase the oscillation visibility while keeping sweep times shorter than T_{2}^{*} using a tailored pulse with a detuning dependent level velocity. Our results show an improvement of a factor of ∼2.9 for the oscillation visibility. In particular, we were able to obtain a visibility of ∼0.5 for Stückelberg oscillations, which demonstrates the creation of an equally weighted superposition of the qubit states.
    Physical Review Letters 02/2013; 110(8):086804. · 7.73 Impact Factor

Publication Stats

27k Citations
2,351.83 Total Impact Points


  • 1989–2014
    • University of California, Santa Barbara
      • • Department of Electrical and Computer Engineering
      • • Department of Physics
      Santa Barbara, California, United States
  • 2013
    • Universität Konstanz
      • Department of Physics
      Konstanz, Baden-Wuerttemberg, Germany
  • 1989–2013
    • Harvard University
      • Department of Physics
      Cambridge, MA, United States
  • 2006–2012
    • University of California, San Diego
      • Department of Physics
      San Diego, CA, United States
  • 1984–2010
    • Princeton University
      • • Department of Physics
      • • Department of Electrical Engineering
      Princeton, NJ, United States
  • 2005–2009
    • ETH Zurich
      • Laboratory for Solid State Physics
      Zürich, ZH, Singapore
  • 2008
    • Nuremberg University of Music
      Nuremberg, Bavaria, Germany
    • Friedrich-Alexander Universität Erlangen-Nürnberg
      • Institute of Optics, Information and Photonics
      Erlangen, Bavaria, Germany
  • 2007–2008
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, MA, United States
  • 2000–2008
    • Macalester College
      • Department of Physics and Astronomy
      Saint Paul, MN, United States
    • University of Michigan
      Ann Arbor, Michigan, United States
  • 2002–2007
    • Universitätsklinikum Erlangen
      Erlangen, Bavaria, Germany
    • University of California, Berkeley
      • Department of Physics
      Berkeley, MO, United States
  • 1993–2007
    • CSU Mentor
      Long Beach, California, United States
  • 2004
    • University of Florida
      • Department of Physics
      Gainesville, FL, United States
  • 2001
    • Russian Academy of Sciences
      • Institute of Solid State Physics
      Moscow, Moscow, Russia
    • Lawrence Berkeley National Laboratory
      • Materials Sciences Division
      Berkeley, CA, United States
  • 1999
    • Northeastern University
      • Department of Physics
      Boston, Massachusetts, United States
    • Vienna University of Technology
      • Institute of Solid State Electronics
      Vienna, Vienna, Austria
    • Ludwig-Maximilian-University of Munich
      • Center for Nanoscience (CeNS)
      München, Bavaria, Germany
  • 1998
    • University of Vienna
      Wien, Vienna, Austria
    • Florida State University
      • Department of Physics
      Tallahassee, Florida, United States
  • 1995
    • Linköping University
      • Department of Physics, Chemistry and Biology (IFM)
      Linköping, OEstergoetland, Sweden
  • 1992
    • Sandia National Laboratories
      • Semiconductor Material and Device Sciences Department
      Albuquerque, New Mexico, United States
    • University Hospital München
      München, Bavaria, Germany
  • 1985–1990
    • AT&T Labs
      Austin, Texas, United States
  • 1982–1989
    • University of Innsbruck
      • Institut für Experimentalphysik
      Innsbruck, Tyrol, Austria
    • The University of Arizona
      • College of Optical Sciences
      Tucson, Arizona, United States
  • 1980–1985
    • University of Illinois, Urbana-Champaign
      Urbana, Illinois, United States
  • 1983
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany